Walk a quarter mile in mud and each boot weighed 20 lb. Use a wire brush to get it off. It was miserable stuff.
It's amazing stuff. I think a student in Montana wrote a research paper on gumbo. From what I remember, it's clay and volcanic ash (the bentonite).
I don't see it here in Western Oregon, but most of the dirt roads are well maintained and gravel, although we have lots of clay. In Central and Eastern Oregon, I see gumbo. When I was riding motos, it was a concern on our spring and summer trips as a quick shower could stop a moto in its tracks. Same for a car or truck.
Stole this pic from another site.